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Old 11-11-2008, 03:47 PM   #1
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5 Amp fuse opens in air handler when supplemental htr is energized


I have a FB4BNF042 "CARRIER" (S/N 1004A73222) Heat Pump Air Handler, the 5 amp fuse blows when the unit is switch to emergency mode (electric supplemental heater is energized). Unit operates normal on heat mode only (temp above 40F). The unit will also open the 5 amp fuse @ the air handler when the outside temperature falls below 40F, this is when the thermostat energizes the supplemental heater via the white wire from the thermostat. I disconnected the white wire (W2) at the thermostat, to prevent the supplemental heater from getting (24vac) power. By doing this the unit operate fine. I'm suspecting the low voltage (24VAC) relay in the air handler that operate the 220 VAC electric heater is faulty. I need a schematic diagram to diagnose the system correctly and identify parts. Thanks.


Last edited by pbservano; 11-11-2008 at 07:36 PM.
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Old 11-11-2008, 04:14 PM   #2
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5 Amp fuse opens in air handler when supplemental htr is energized


isnt there a small board that controls the heat thats probably where the trouble is or if it has a sequencer the low voltage portion is probably shorted if its out of warranty u could just perform a little surgery and put a contactor in its place depending on how many kw it is

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Old 11-11-2008, 08:48 PM   #3
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5 Amp fuse opens in air handler when supplemental htr is energized


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Originally Posted by pbservano View Post
I'm suspecting the low voltage (24VAC) relay in the air handler that operate the 220 VAC electric heater is faulty.
I could find coil resistance data for only one relay of this type and it was 77 ohms, so pass/fail resistance for a relay like this may be 50 to 150 ohms.
Also check that the coil isn't shorted to the relay frame.

Is the fuse in a 120v line?

Last edited by Yoyizit; 11-11-2008 at 08:59 PM.
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Old 11-12-2008, 01:53 AM   #4
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5 Amp fuse opens in air handler when supplemental htr is energized


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Originally Posted by Yoyizit View Post
I could find coil resistance data for only one relay of this type and it was 77 ohms, so pass/fail resistance for a relay like this may be 50 to 150 ohms.
Also check that the coil isn't shorted to the relay frame.

Is the fuse in a 120v line?

The fuse is in the control board, 24VAC, that controls the air handler blower. This air handler is a three year old CARRIER unit. There is a relay that has 4 diodes that controls the electric heater. Carrier uses a different type of relays to energize the heater. I need to identify this relay so I can order a replacement.

Last edited by pbservano; 11-12-2008 at 02:00 AM.
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Old 11-12-2008, 07:27 AM   #5
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5 Amp fuse opens in air handler when supplemental htr is energized


Anyone here has the schematic diagram of the control board for CARRIER indoor unit, Model number, FB4BNF042.

I suspect that the wiring on this control board was incorrect. My neighbor said that a "preventive maintenance" was performed last August and the tech went up the attic and worked on the air register. This guy was passing out flyer for a $50 check-up. I went through his entire unit, this guy missed a loose connection at the condensing unit contactor, causing a burnt power lead wire for the compressor. Tested the 24 VAC contactor test sat. Another incorrect wiring was performed at the thermostat terminal board, the white wire was connected to the "B" terminal and the blue wire was connected to the "W" terminal. When I corrected the wiring the thermostat quit working. This is the contractor type thermostat, non programmabble. Since, i worked as a HVAC tech I have lots of spare thermostat in my truck so I replace it with a new thermostat.During functional test the fuse blows only when the white wire is connected to "W" and operate normally when disconnected. The unit worked flawlessly last winter, now he has issue with the fuse opening @ the indoor unit. While troubleshooting at the indoor unit, I noticed the white wire was connected to the terminal board marked "C" (brown wire) on the control board. Is this suppose to be connected to blue wire, which is common for 24VAC control system? Just trying to help a 86 year old neighbor that has fixed income and cannot afford to pay a tech. I disconnected the white wire at the thermostat, so he can have heat, the heat pump operate fine with this wire disconnected. My e-mail address is pbservano@aol.com. Thanks a lot.

Last edited by pbservano; 11-12-2008 at 07:35 AM.
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Old 11-12-2008, 07:57 AM   #6
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5 Amp fuse opens in air handler when supplemental htr is energized


do yourself a favor since it sounds like you know what your doing remove all or whatever wires are questionable in terms of colors at the stat, outdoor ,and air/hand and just rewire it the way you want it black or blue for common and it seems to me that "B" isnt even used shouldnt that be "O" is been a couple years for me but if i remember correctly "B" is to operate the reversing valve in heat instead of a/c in other words the r/v is normally powered for a/c so if it fails it fails in heat (if i remember correctly) someone correct me if im wrong i think rheem/ruud are one of the only manu. that power in heat instead of a/c
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Old 11-12-2008, 08:51 PM   #7
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5 Amp fuse opens in air handler when supplemental htr is energized


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Originally Posted by kennzz05 View Post
do yourself a favor since it sounds like you know what your doing remove all or whatever wires are questionable in terms of colors at the stat, outdoor ,and air/hand and just rewire it the way you want it black or blue for common and it seems to me that "B" isnt even used shouldnt that be "O" is been a couple years for me but if i remember correctly "B" is to operate the reversing valve in heat instead of a/c in other words the r/v is normally powered for a/c so if it fails it fails in heat (if i remember correctly) someone correct me if im wrong i think rheem/ruud are one of the only manu. that power in heat instead of a/c

Re-wired the wiring in the indoor unit, including the wires in the condensing unit. I also found the white wire was also connected at the common side of the defrost circuit board and the blue wire to the terminal board marked "W", I corrected the wiring. Energized the emergency heat at the thermostat, fuse did not blow. My neighbor also complained about blowing the fuse during about midnight and sometime 4 in the morning. I assume that the temp. outside went below 40F and the unit went into defrost mode, thereby energizing the electric heat. I performed a functional test to initiate defrost while the unit was running in heat mode. Fuse did not blow this time. I think I got this issue resolved. Thanks alot for all the help. This site is great!

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